1. Field of the Invention
This invention relates to a polarization maintaining optical fiber which is useful in the field of communication or in the field of sensors using optical fibers, and relates to a method for producing an optical fiber preform for producing optical fibers.
2. Description of Related Art
There are various types of the polarization maintaining optical fibers, among which a stress applied type polarization maintaining optical fiber including a stress applying portion in a cladding portion are well known. Specific examples of the stress applied type polarization maintaining optical fiber includes a PANDA type polarization maintaining optical fiber, a bow-tie type polarization maintaining optical fiber, an elliptical jacket type polarization maintaining optical fiber, and the like, of which respective cross-sectional shapes are different from each other. Among these types, the PANDA type polarization maintaining optical fiber has been widely used because it has a low transmission loss and superior polarization maintaining characteristics.
FIG. 5 shows a conventional PANDA type polarization maintaining optical fiber according to a prior art. The polarization maintaining optical fiber 4 includes a core 1 having a high refractive index, a cladding 2 having a low refractive index and disposed around the core 1 so as to be concentric with the core 1, and two stress applying portions 3 and 3 having a circular cross-sectional shape and a refractive index generally lower than that of the cladding 2 and disposed in the cladding 2 so as to be symmetrical with respect to the core 1.
The stress applying portion 3 is produced from a material having a relatively large coefficient of thermal expansion. Therefore, when the polarization maintaining optical fiber 4 is produced by melting and drawing an optical fiber preform, the core 1 is subjected to different degrees of stress from transverse and longitudinal directions during glassification, as a result of which large distortions are anisotropically generated in the core 1, which causes birefringence in the polarization maintaining optical fiber 4.
In order to produce the polarization maintaining optical fiber 4, an optical fiber preform 14 including a core element 11 for producing the core 1 and a cladding element 12 for producing the cladding 2, which are shown in FIG. 6, are prepared. Then, a pair of insertion holes is formed in the cladding element 12 so as to be symmetrical with respect to the core element 11 and to pierce through the optical fiber preform 14 in the longitudinal direction, into which stress applying elements for producing the stress applying portions 3 are inserted. Then, the optical fiber preform 14 is melted and is drawn to produce the polarization maintaining optical fiber 4 shown in FIG. 5.
The insertion holes for disposing the stress applying elements in the cladding element 12 are conventionally formed by forming one insertion hole 13a by means of a drilling tool 16 such as a drill or a grindstone, followed by forming the other insertion hole 13b after parallelly transferring the drilling tool 16 to the position symmetrical with respect to the core element 11 in which the insertion hole 13b is formed, as shown in FIG. 6.
As shown in FIG. 7, when the insertion hole 13a is formed in a position slightly vertically off the transferring direction of the drilling tool 16, the insertion holes 13a and 13b and the core element 11 are not arranged in a line. An angle A is thereby formed by a line connecting the center of the insertion hole 13a with the center of the core element 11 and a line connecting the center of the insertion hole 13b with the center of the core element 11 (which is referred to as xe2x80x9cangle disparityxe2x80x9d, hereinafter).
When the optical fiber preform 14 is melted and is drawn while having the angle disparity, the produced polarization maintaining optical fiber 4 also has the angle disparity.
When two polarization maintaining optical fibers 4 and 4 are fused after adjusting end faces thereof or after arranging in a line, a controlling step in which the polarization maintaining optical fiber 4 is circumferentially rotated is required so as to suppress deterioration of polarization crosstalk characteristic of the fused polarization maintaining optical fiber. In the controlling step, one of two polarization maintaining optical fibers is circumferentially rotated until each position of the core and the stress applying portions of the polarization maintaining optical fiber is adjusted to each position of the core and the stress applying portions of the other polarization maintaining optical fiber while observing the polarization maintaining optical fibers from their side. Hereinafter, the controlling step is referred to as a step of xe2x80x9cpolarization axis matchingxe2x80x9d. At this time, it is required that the stress applying portions and the core be arranged in a line so as to carry out the polarization axis matching without deteriorating the polarization crosstalk characteristic. However, since the polarization maintaining optical fiber produced by the conventional method has the angle disparity, the polarization crosstalk characteristic is deteriorated after fusing at least two polarization maintaining optical fibers.
An object of the present invention is to provide a polarization maintaining optical fiber in which deterioration of polarization crosstalk characteristic is suppressed after fusing at least two polarization maintaining optical fibers and to provide a method of producing a polarization maintaining optical fiber preform used for producing the polarization maintaining optical fiber.
The present invention provides a polarization maintaining optical fiber comprising: a core; a cladding disposed around the core; and two stress applying portions disposed in the cladding to be approximately symmetrical with respect to the core, wherein an angle formed by a line connecting the center of one of the stress applying portions with the center of the core and a line connecting the center of the other stress applying portion with the center of the core is 3 degrees or less.
Moreover, the present invention provides a polarization maintaining optical fiber preform comprising: a core element; a cladding element disposed around the core element; and two stress applying elements disposed in the cladding element to be approximately symmetrical with respect to the core element, which is produced by forming an optical fiber preform including the core element and the cladding element, forming one insertion hole in the cladding element so as to pierce through the cladding element in parallel to the core element, and then rotating the optical fiber preform further including the insertion hole 180 degrees around the core element, followed by forming the other insertion hole in the cladding element so as to pierce through the cladding element in parallel to the core element, and then inserting the stress applying elements into the insertion holes.
The aforementioned polarization maintaining optical fiber may be produced from the aforementioned polarization maintaining optical fiber preform.
Moreover, the present invention provides a method of producing a polarization maintaining optical fiber preform including a core element, a cladding element disposed around the core element, and two stress applying elements disposed in the cladding element to be approximately symmetrical with respect to the core element, comprising: a step of forming an optical fiber preform including the core element and the cladding element; a step of forming one insertion hole in the cladding element so as to pierce through the cladding element in parallel to the core element; a step of rotating the optical fiber preform further including the insertion hole 180 degrees around the core element; a step of forming the other insertion hole in the cladding element so as to pierce through the cladding element in parallel to the core element; and a step of inserting the stress applying elements into the insertion holes.
Moreover, the present invention provides a method of producing a polarization maintaining optical fiber comprising a step of melting and drawing the aforementioned polarization maintaining optical fiber preform.